CAPE CANAVERAL, Fla. (AP) -- NASA has cleared Discovery for a Monday morning launch to the International Space Station, the last scheduled liftoff in darkness for the soon-to-retire shuttle program. Mission managers met Saturday and gave the ``go'' to proceed toward liftoff. The unanimous decision came after engineers determined there were no booster rocket safety concerns. Two booster problems recently cropped up when the parts were tested offsite. Forecasters expect an 80 percent chance of good weather for the 6:21 a.m. liftoff, close to an hour before sunrise. Even though Sunday is Easter, countdown preparations will continue as usual, with the same number of workers. Launch director Pete Nickolenko said he asked his team if anyone had any conflicts because of the holiday, and no one did. Discovery is loaded with spare parts and science experiments for the space station. A crew of seven will deliver all the gear and conduct three spacewalks to set everything up. On Saturday, the astronauts got their customary preflight haircuts. ``Pre-launch haircut complete ... High and tight!'' balding crew member Clayton Anderson wrote in a Twitter update. He said some of his Marine astronaut buddies ``would be proud!'' Only three shuttle flights remain after this one. President Barack Obama will visit the Cape Canaveral area April 15 -- while Discovery is in orbit -- to elaborate on his post-shuttle plans. He created a furor in the aerospace community in February when he killed NASA's Constellation program, which had been aimed at returning astronauts to the moon. That will mean even more lost jobs for Kennedy Space Center and NASA's other hubs for human spaceflight operation. Launch manager Mike Moses told reporters that even as the shuttle program winds down, the work force remains as loyal and dedicated to the job as ever. ``But I don't want to take away from the fact that this is a very human space program, not just with the humans flying in the shuttle, but the folks building it and preparing it and getting ready to launch it,'' he said. As excitement builds toward Monday morning's launch, ``I don't think there are too many people out there right now at their desks, worried that we're about to end,'' Moses said. ``You ask that question on Tuesday, we might get a little different answer. But right now, I think spirits are very high and geared up toward that launch.'' Discovery will spend 13 days in orbit, on its next-to-last flight.